In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of an assembly of drill pipe sections that are connected end-to-end (commonly referred to as a “drill string”), and then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. In conventional vertical borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or, in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel).
During the drilling process, a drilling fluid (also commonly referred to in the industry as “drilling mud”, or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the borehole, and then upward back to the surface through the annular space between the drill string and the wellbore. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry borehole cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that clean the bit's cutting elements and blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the borehole wall (to stabilize and seal the borehole wall).
It has become increasingly common and desirable in the oil and gas industry to drill horizontal and other non-vertical boreholes (i.e., “directional drilling”), to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical boreholes. In directional drilling, specialized drill string components and “bottom hole assemblies” are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of desired non-vertical configuration.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “drilling motor” or “mud motor”) incorporated into the drill string immediately above the drill bit. In drilling processes using a downhole motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. However, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the downhole motor to generate power to rotate the drill bit.
In directional drilling, the path of the drill bit is typically deviated in a desired direction by means of a bent housing or a bent sub, typically disposed within downhole motor. Bent subs and bent housings serve the same purpose, and in general terms differ only in that a bent housing is adapted to accommodate a drive shaft through its central bore. Bent subs and bent housings may be fashioned with a fixed or adjustable bend angle. The motion of a drill bit rotating in conjunction with a bent housing is complex, and consequently cannot be described using a simple helical model.